The design of pixels in imaging electronics is generally well-known by those of ordinary skill in the art. One commonly employed CMOS active pixel sensor (APS) pixel architecture is referred to as a “three-transistor (3T) design” that, as the name implies, includes a first transistor M1, a second transistor M2, and a third transistor M3.
Transistor M1 is a reset transistor that provides a path to charge the photodiode node. When transistor M1 is on, the photodiode node is connected to a power supply signal (e.g., V+). Transistor M2 is a buffer transistor that acts as a voltage buffer between the photodiode node and a common bit line. The photodiode node is read to a common bit line. Since the gate of the transistor M2 is high impedance, the voltage at the photodiode node may be read onto the bit line without affecting the voltage on the photodiode node. Transistor M3 is a row selected device that multiplexes multiple pixels to the common bit line. Transistor M3 of each pixel selectively connects each pixel to the common bit line in a sequential fashion.
A bright signal typically bleeds charge from the photodiode node more quickly than a dark signal. After an integration period, the difference between the integration level and the reset level represents the amount of light that is received by the pixel. When the difference is small, the resulting pixel in the image is relatively dark. Similarly, when the difference is large, the resulting pixel in the image is relatively bright.
An analogy to pixel operation is a bucket of water, where the water represents the charge on the photodiode node. The bucket has a hole, whose size represents the intensity of light received by the pixel. When the size of the hole in the bucket is large (representing a large light intensity), the water (representing charge on the photodiode node) escapes more quickly from the bucket in a given time period (referred to as “integration period”). When the size of-the hole in the bucket is small (representing a small light intensity), the water (representing the charge on the photodiode node) escapes more slowly from the bucket in a given time period (e.g., an integration period). The amount of water that has escaped the bucket in a given time period represents the amount of light received by the pixel during the integration period. As can be appreciated, a proper reset (e.g., filling the bucket to a predetermined level in the above analogy) is important to obtain the proper signal or meaningful value.
For example, when the bucket has no bottom, the bucket cannot be filled to a predetermined level (resetting the photodiode node). In this case, since there is no water in the bucket, there is no meaningful measurement of the amount of water that escapes the bucket since the difference is always zero. This is commonly referred to as a “black sun” problem since the sun, which is a Very bright object, in the scene often causes black spots in the resulting image, which is an undesirable artifact.
One approach to solving the “black sun” problem is to increase the size of the reset pixel. In this manner, as the width of the reset transistor is increased to larger values (e.g., 10 microns), the photodiode node of the pixel can be more readily reset even under bright light conditions. However, since the trend is for digital cameras to offer multiple megapixel imagers, increasing the size of the reset transistor is not a feasible solution.
Based on the foregoing, there remains a need for a method and apparatus for detecting failure to establish a reset signal in pixels of an imager that overcomes the disadvantages set forth previously.